Cellular senescence is a state of irreversible growth arrest controlled by the p53 and retinoblastoma (Rb) tumor-suppression pathways. To become cancerous, a cell must bypass its intrinsic senescence signals. Despite the fact that the p53 and Rb pathways are disrupted in many types of cancer, tumor cells appear to retain the capacity to senesce. Several current cancer therapies such as treatment with doxorubicin, a widely used chemotherapy drug, have been shown to trigger senescence in vitro and in vivo. In a study of breast cancer patients, senescent cells were found specifically in tumor tissues of women who had undergone chemotherapy as compared to those who had not. These and other lines of investigation suggest that induced senescence could be a viable basis for new cancer treatments. To effectively exploit senescence as a therapeutic mechanism, a better understanding of cellular genes involved in inducing and establishing various aspects of the senescent phenotype in cancerous cells is necessary. This study seeks to identify cellular genes required for senescence. A cell-based system for examining senescence in human cancer cells has been established and extensively characterized in the lab. A clonal line of HeLa cells was engineered to express the human papillomavirus (HPV) 16 E6 gene (HeLa-E6 cells). Expression of bovine papillomavirus (BPV) E2 represses transcription of the endogenous HPV18 E6 and E7 genes in HeLa-E6 cells without affecting HPV16 E6 expression. Repression of E7 results in a uniform senescence response via reactivation of the Rb pathway, while the p53 pathway remains blocked by the HPV16 E6 protein. Studies proposed here will employ RNA interference library screens to uncover genes important for Rb pathway-mediated senescence using this system. Further experiments will assess the ability of candidate genes to block senescence induced by other mechanisms in various cell types and analyze their individual roles in establishing specific aspects of the senescent phenotype. Additional analyses will be designed according to putative target gene identity to examine the mechanisms through which senescence is blocked by individual candidate gene repression. Information gained in this study will be important for the future development of senescence-based cancer treatments potentially effective against a wide range of cancers. The second leading cause of death (CDC, 2004), cancer affects people of all ages, races, and gender. According to the American Cancer Society, in the year 2007 more than 1,500 people in the United States will die each day from this disease. Work proposed here seeks to gain information that will help direct the development of novel cancer therapy strategies.